Linear integration of spine Ca ²⁺ signals in layer 4 cortical neurons in vivo

Abstract

Sensory information reaches the cortex through synchronously active thalamic axons, which provide a strong drive to layer 4 (L4) cortical neurons. Because of technical limitations, the dendritic signaling processes underlying the rapid and efficient activation of L4 neurons in vivo remained unknown. Here we introduce an approach that allows the direct monitoring of single dendritic spine Ca 2+ signals in L4 spiny stellate cells of the vibrissal mouse cortex in vivo. Our results demonstrate that activation of N-methyl-D-aspartate (NMDA) receptors is required for sensory-evoked action potential (AP) generation in these neurons. By analyzing NMDA receptor-mediated Ca 2+ signaling, we identify whisker stimulationevoked large responses in a subset of dendritic spines. These sensory-stimulation-activated spines, representing predominantly thalamo-cortical input sites, were denser at proximal dendritic regions. The amplitude of sensory-evoked spine Ca 2+ signals was independent of the activity of neighboring spines, without evidence for cooperativity. Furthermore, we found that spine Ca In the mammalian brain, sensory information is transmitted to the neocortex primarily through thalamo-cortical projections. Thus, in the vibrissae-related somatosensory cortex of rodents, tactile information arrives through axons of neurons that are located in the ventro-posterio-medial thalamic nucleus (VPM). It is well established that these axons provide direct input to layer 4 (L4), where they contact the dendrites of three types of excitatory neurons, namely the spiny stellate cells, the pyramids, and the star pyramids (1). In addition, there is recent evidence that axons originating in the VPM provide a direct input also to layer 5 (L5) neurons (2). A particularly interesting cytoarchitectonical feature of L4 of the rodent vibrissal cortex is the organization in distinct structures called "barrels" (3). The dendritic arborization of each spiny stellate cell is confined within the border of a barrel and is typically asymmetric, with cell bodies located at the barrel's periphery and the dendrites facing the center (4, 5). Functionally, most spiny stellate cells and also the other L4 neurons in each barrel respond primarily to a single whisker, referred to as the principal whisker, whereas adjacent, or surround, whiskers provide a much weaker input (6). The fraction of thalamocortical inputs to each L4 neuron is remarkably small (about 10-20%) compared with that of intracortical inputs (7). Nevertheless, thalamocortical inputs are very efficient because of their synchronous activation by sensory stimuli (8, 9).The dendritic processes that are associated with the synaptic activation of L4 neurons were investigated initially in vitro in acute brain slices. For example, a study involving the use of whole-cell recordings in combination with two-photon imaging characterized synaptically evoked N-methyl-D-aspartate (NMDA) receptormediated Ca 2+ influx in dendritic spines (10). The analysis of the spike timing dependence of the Ca 2+ trans...

Dendrites Enable a Robust Mechanism for Neuronal Stimulus Selectivity

“…In contrast, for 35 synapses distributed across the seven branches, scattered stimulation depolarizes the soma more than the same number of clustered synapses because their activation generates multiple NMDA spikes (Fig. 5C, red line) as has been observed experimentally in vivo (Jia et al, 2014). These observations are summarized in an expected/measured plot ( Fig.…”

Deep two-photon brain imaging with a red-shifted fluorometric Ca 2+ indicator

“…The single-photon excitation and emission peak wavelengths of Cal-590 are 570 and 590 nm, respectively. For the use of Cal-590 in in vivo two-photon experiments, we adapted components of a high-speed resonant scanner-based two-photon microscope (15) for measurements at long excitation wavelengths (>1,000 nm, Fig. 1A) and determined the two-photon excitation spectrum of Cal-590 using a laser with a broad tuning range (680-1,300 nm; for details, see Materials and Methods).…”

“…Z-stack images of the labeled neuron or the neuronal population were obtained with a step size of 0.5 μm, at an imaging frame rate of 40 Hz (600 × 600 pixels), by recording 40 images at each focal plane. Data acquisition was controlled by custom-made software written in LabVIEW 2014 (National Instruments) (15). .…”

From single cells and single columns to cortical networks: dendritic excitability, coincidence detection and synaptic transmission in brain slices and brains

“…; Jia et al . ) and (ii) spiking probability data from whole‐cell and extracellular loose‐patch unit recordings (de Kock et al . ) or only indirectly, by calcium fluorescence imaging data of active cell somata.…”